The present invention relates to analogue selectors, also known as analogue multiplexers, especially though not exclusively as implemented in integrated circuits for audio equipment such as hi-fi amplifiers.
Analogue input selectors have many applications, including for example in audio entertainment equipment such as home hi-fi, in-car entertainment and portable equipment which normally provide for a choice of audio sources such as a CD player, tape deck or radio for example. Such equipment also typically routes audio input signals to one or more outputs, for example headphones, loudspeakers, tape recorders and digitisers. This is typically implemented using a bank of input selectors, one for each output.
With increasing equipment miniaturisation, analogue selectors are typically implemented as Integrated Circuits (IC) utilising MOSFET switching technology. MOSFET""s such as CMOS are used because of their low leakage current compared with other types of transistor switch technology. FIG. 1 shows a typical circuit arrangement for an analogue input selector having programmable gain. Programmable or variable input gain is used because of the different signal levels provided from the different types of analogue sources. For example a record deck might provide a few millivolts rms whereas a standard xe2x80x9clinexe2x80x9d output from audio equipment provides typically 2V rms. A CMOS selector switch is provided for each input source, and the CMOS gain selector switches are arranged according to what signal level the selected input source delivers. The output signal level of the analogue selector should therefore be similar irrespective of the input source.
A problem with this arrangement is that the use of CMOS type switches in the IC results in signal level dependent distortion as illustrated in FIG. 2. This is because the resistance of the CMOS switch is dependent on the voltages from drain and source to the gates, which voltages are not constant in the selector switches because the drain and source voltages rise and fall with the signal whereas the gate voltages are related to the supply rail voltages which are fixed.
A solution to this problem is to increase the size of the selector switch in order to reduce its input impedance relative to other, constant, series impedances in the circuit, and hence reduce the voltage across the drain/source, and hence reduce the relative effects of any modulation of the CMOS switch resistance. However larger switches require more IC real-estate which increases cost and also increases undesirable parasitic capacitances, which can degrade power-supply rejection for example.
Another proposed solution to this problem is described in M. Sato, K. suzuki, T. Suzuki, and K. Akutsu, xe2x80x9cA Volume and Tone Control IC for Hi-Fi Audioxe2x80x9d, IEE J Solid-State Circuits, Vol SC-16, No. 6, December 1981. p682-688. This document describes the use of very high sheet resistance material for the gate electrodes. The local gate Voltage is then capacitively coupled to the channel at signal frequencies to maintain an approximately constant gate-channel voltage. But this requires unusual manufacturing steps, which increases cost and such processing is not readily available. It also introduces delays when switching from one channel to another and so this idea has not been adopted.
A further solution is described in T. L. Brooks et al xe2x80x9cA Cascaded Sigma-Delta Pipeline A/D Converter with 1.25 MHz Signal Bandwidthxe2x80x9d in IEEE Journal of Solid-State Circuits, Vol.32, No.12, December 1997xe2x80x94see especially pages 1901-2. This uses a charge pump architecture which effectively boosts the gate voltage above the supply rail depending on a previously sampled signal level in order to try to maintain a substantially constant source/drain to gate voltage difference. However this architecture introduces a requirement for high speed clocking which can itself be a source of distortion and switching transients added to the signal. The extra complexity of the circuitry will also increase chip area, and hence cost. There is also some reliability risk, both for the switch and whatever circuit drives the gate drive, involved in charge-pumping the voltages possibly higher than the normal supply voltage
In general terms in one aspect the present invention aims to provide an analogue selector in which the gate to source or drain voltages of transistors such as MOSFETs used as semiconductor switches are held constant by holding the source or drain terminals at a fixed or signal independent voltage, such as virtual earth. This means that these voltages do not vary as the signal voltages vary. This is achieved by replacing the selection switch coupled between each input connection and a gain stage with a gain stage for each input connection. The gain stage for the chosen input channel operates with one of its constituent MOSFET switches turned on and connected to a virtual earth or other supply related voltage in order to provide a substantially constant source and drain to gate voltage. By removing the floating voltage selection switches, distortion caused by the switch resistance varying with the source and drain to gate voltages is substantially eliminated.
For the purposes of this specification, the terms drain, source and gate are used for simplicity and clarity of explanation, but are intended to encompass corresponding structures having similar functions in non-FET switches. For example the terms drain, source and gate are intended to encompass the collector, emitter and base structures in bipolar junction transistors as well as similar structures with the same function in other types of semiconductor devices.
In particular in one aspect the present invention provides an analogue selector according to claim 1.
Such an arrangement provides that the input coupling stage of the analogue selector does not require the selector switch for each input and thus a significant source of distortion is removed. The switching function is now performed by the input gain stages or controllers. Whereas large selector switches are typically used in order to reduce signal level dependent distortion, and their physical size is further increased by layout precautions to allow them withstand directly applied ESD and latch-up stresses, removal of these by the present arrangement also saves valuable IC real-estate.
Preferably the gain stages or controllers comprise one or more semiconductor switches each having a source or drain coupled to the virtual earth of the selector.
This arrangement provides the switches with a fixed source and drain to gate voltage as the virtual earth is related to the supply rail voltages and not the signal voltages. Such an arrangement takes up relatively little IC real-estate as small switch configurations can be used because the switches carry substantially no signal current and are not directly exposed to external ESD or latch-up stresses
In one embodiment the selector comprises an amplifier such as an op amp having an input and an output, and each gain stage or controller further comprises an impedance chain coupled between the amplifier output and a respective input connection, the switches being coupled between the chain and the amplifier input. Alternatively each gain stage may comprise a dedicated amplifier such that the whole op amp input stage is duplicated per-channel and switched in as required.
An impedance chain is typically implemented as a series of discrete impedance devices such as resistors, from which taps of different combined impedance can be taken at different points along the chain, for example at the connections between discrete impedances.
This allows multiple gain factors to be selected using a simple impedance chain such as a series of resistors, the switches being coupled between resistor connections and the amplifier input. If the switches are all left open (zero gain), this deselects the input signal.
In an alternative arrangement, one or more of the gain controllers may comprise just one switch with impedance coupled between the switch the respective input and further impedance coupled between the switch and the amplifier output. Such an arrangement provides just two gain settings, zero (off) and non-zero (on); and thus acts as a simple input selector without adjustable non-zero gains.
Preferably the selector further comprises means for shorting each of the gain stages to a reference voltage when at zero gain. This reduces the possibility of any cross-talk between the input circuits.
Preferably the shorting means comprises one or more switches coupled between each impedance chain and a reference voltage. Preferably the amplifier has a second input connected to the reference voltage.
When implemented as an integrated circuit, preferably the shorting means further comprises another switch which is independently routed from said other shorting means switches to said reference voltage.
Since the selector switches pass substantially no signal current, they can be physically small. The resistance requirements on the shorting switches are also modest, since these are not in the direct signal path. In both cases an NMOS or PMOS switch is adequate, which take up little IC real-estate.
In a second aspect the present invention provides a multiplexed programmable gain amplifier. This comprises a plurality of input connections for coupling to a number of input sources. Each input connection is connected to a programmable gain stage having at least zero and one or more non-zero gains. Preferably each programmable gain stage has one or more semiconductor switches each having a non-gate input coupled to the virtual earth of the selector. Preferably each programmable gain stage has an impedance chain which is coupled between an amplifier output and a respective input connection. The switches are coupled between the chain and the input of the amplifier.
Further implementations and advantages of the present invention will become apparent from the following description of embodiments.